Field of the Invention:
The present invention relates to a method and a device for the spectral analysis of a metal coating layer deposited on the surface of a steel strip.
The invention relates in particular to the quality control of a metal coating deposited by dipping a continuously moving metal strip, for example a zinc-alloy coating having undergone an alloying process.
Hot dip galvanizing of continuously moving rolled steel strips is a well-known technique. It essentially comprises two variants: one where the strip leaving the galvanizing furnace descends obliquely into a bath of molten zinc and is diverted vertically upwards by a roll immersed in said molten zinc. The other variant involves diverting the moving steel strip vertically upwards as it leaves the furnace, before passing it through a vertical channel containing molten zinc sustained magnetically.
The formation kinetics of this deposition would be known to a person skilled in the art and has been disclosed on numerous occasions, including in “Modeling of galvanizing reactions” by Giorgi et al. in “La Revue de Métallurgie CIT” in October 2004. This documentation establishes that contact between the molten coating mixture and the strip causes dissolution of the iron from the steel strip that, firstly, participates in the formation, on the surface of the strip, of a compound layer of approximately 0.1μ of the compound Fe2Al5Znx and, secondly, spreads to the bath of molten mixture until the Fe2Al5Znx layer has formed continuously. The Fe2Al5Znx layer serves to support the protective zinc layer while the dissolved iron contributes to the formation of precipitates comprising Fe, Al and Zn, known as “dross”, in the molten mixture.
The coating of steel strips with a zinc-based metal alloy has numerous applications, for example in car bodywork, domestic appliances and construction. Several coatings stand out on account of their composition and possible treatment once applied to the strip.
One such coating undergoes an “alloying” process by reheating, which uses diffusion to form an alloy between the iron in the steel and the zinc in the coating. This type of coating is generally referred to by the abbreviation “GA” or the term “galvanneal”. GA galvanizing provides good protection of the steel against corrosion and certain specific qualities, such as the porosity and surface texture thereof, making it moreover particularly suitable for paint, of which it increases the adherence, and facilitating resistance welding of the coated strips. For these reasons, it is primarily used in the automotive industry.
The iron/zinc alloy of a GA coating may be provided in the form of several phases having different crystallographic meshes and compositions. In order to ensure that the coating provides optimum characteristics for deep drawing and painting, studies have shown that the iron content in the surface of the coating must be low without reaching zero. Numerous parameters are liable to affect the microstructure of the galvanneal layer—the chemical composition of the strip and the chemical composition of the coating bath, the surface roughness of the strip, the temperature of the coating bath and, above all, the temperature in the alloying furnace and the speed of movement of the strip.
Studies have been carried out to define means for tracking the development of zinc, iron and aluminum content through the thickness of the galvanneal deposits in order to better understand the influence of each parameter and to monitor the quality of the coated strips. Several methods for determining the concentration profile of elements of alloys on samples of coated strips have been proposed, including “Glow discharge optical emission spectroscopy GD-OES” and “Laser-induced breakdown spectroscopy LIBS” spectroscopic measurement.
Document WO 00/08446 discloses the basis for such an analysis method using laser ablation (LIBS) and document US 2003/0016353 provides a number of improvements intended to increase the accuracy of measurements.
Nonetheless, the analysis methods described are very difficult to implement for use in real time on a galvanizing line where the strip is moving at speeds of up to more than 3 meters per second.
Indeed, in a known manner, the LIBS method uses the interaction between laser and matter to heat a target to be analyzed to a very high temperature and to generate spectral lines characteristic of the material heated at the target point of the strip. A pulsed laser is therefore fired at the target for a very short time, around 10 nanoseconds, at a power of several tens of millijoules aimed at a very weak surface. A micro-plasma is created that emits a continuum of radiations and, after a few microseconds, the first spectral lines appear, said lines being characteristic of the material of which the target, and therefore the strip coating, is made. A spectrometer is used to perform a spectral analysis of the light emitted during a measurement period during the microseconds following the laser shot, when the background noise of the continuum is sufficiently reduced.
During a single microsecond's delay between the laser firing and the spectrometer measurement, and for a movement of around 3 meters per second, the strip and therefore the target and the plasma are moved 3 millimeters. To take a measurement on a moving target, a person skilled in the art would create a positional offset between the firing axis of the laser and the optical axis of the measurement device, which is very difficult to achieve with sufficient precision, especially as the speed of movement of the strip during galvanizing can vary significantly as a function of the thickness of said strip, the heating capacity of the galvanizing furnace, the thickness of the coating in question, and the performance of the devices for drying the liquid coating.
Furthermore, intrinsic vibrations or deformations in the strip or the trajectory of movement thereof make said measurement very inaccurate, given that a local targeting precision of a few microns is required. An article entitled “New approach to online monitoring of the Al depth profile of the hot dip galvanized sheet steel using LIBS”, H. Balzer et al., published on Mar. 29, 2006, Springer Verlag 2006, discloses for this purpose a sample system using complex vibration offsetting. This article mentions that this system is designed for low strip movement speeds (less than 1 m/s) while the speeds required in an industrial context may be around 3 to 4 m/s. Such a system would therefore be unable to offset the resulting vibrations.